1. Field of the Invention
This invention relates to a video signal stretcher of a type suitable for stretching a range of a video signal reproduced from storage media such as a video disc, video tape and the like.
2. Description of the Prior Art
In NTSC (National Television System Committee) color television system, a waveform of a composite picture signal (see FIG. 8) is standardized by a standard in which, by assigning a pedestal level to 0 V, a white peak level is set to +0.715 V and that a synchronizing signal level to -0.285 V, whereas a peak to peak voltage conforms to 1 Vp-p. Apart from this type of expression, the standard may be expressed in IRE scale, wherein the pedestal level is 0 IRE, the white peak level is +100 IRE and the synchronizing signal level is -40 IRE.
In the NTSC color television system, a black level was set to 7.5.+-.0.25 IRE which is slightly higher than 0 IRE, however, the IRE scale has been revised recently to a range of 0-100 IRE. In accordance with this revision, the average black level is now set to 5.0 IRE which is approximately 0.04 V in MKSA system of units.
In existing standards for video software, two types of standards for video software are common on the market, whereas one type of video software is a conventional video signal of 7.5 to 100 IRE whilst the other type of video software is a new video signal of 0-100 IRE. For this reason, if a conventional video software is reproduced by a reproducer arranged for new video software, a reproduced video signal may be displayed on a picture screen as a whitened picture with high brightness. In this case, if a black level of the reproduced video signal is simply lowered from 7.5 IRE to 0 IRE, the reproduced video signal may vary its level within a range of 0 to 92.5 IRE, thus a white level is lowered and results in a dark image on the picture screen.
It is therefore an object of this invention to provide a video signal stretcher wherein a set-up black level of a video signal is brought to coincide with a pedestal level thereof by stretching the video signal.